This invention relates to a process and an apparatus for producing ultrafine particles of metals, metallic compounds and ceramics with high purity and controlled particle shape, particle size and particle size distribution by a plasma method in order to yield ceramic sintered bodies with highly controlled microstructures and constructions.
Ceramic ultrafine particles or powders are used for giving structural, functional or biotechnological ceramic materials (sintered bodies). With recent progress in studies, in order to meet requirements for fine ceramics partly used practically, for example, in order to obtain high physical properties such as high heat resistance, high strength, high toughness, etc., in the case of structural materials such as engine parts in cars, or in order to obtain uniformity in physical properties in the case of functional materials such as chemical sensors, there have been desired to have uniform grain sizes in ceramic sintered bodies in the range of about 0.5 to 5 .mu.m, to have pores with a uniform size and distributed uniformly in ceramic configuration, or to have no pores depending on use, to have impurities in the crystal grain boundaries as small as possible, or to be controlled to have constant components.
Ceramic raw material powders can be produced by a grinding and classifying method, a wet method with chemical substances, or a gas phase method wherein fine particles are formed by synthesis by a dry method.
According to the grinding and classifying method, there are defects in that impurities are easily mixed, the resulting particles are angular and easily form large spaces irregularly when molded, it is difficult to classify raw material particles with uniform particle size in the range to 0.05 to 1.0 .mu.m, which range is considered to be preferable in the present ceramic production technique to give crystal particles with most uniform in quality, and dense, small and uniform particle size, and particularly there is obtained a broad particle size distribution.
According to the wet method, in the case of oxide ceramics such as Al.sub.2 O.sub.3, SiO.sub.2, ZrO.sub.2, etc., there can be obtained spherical particles with the desired particle size directly, or primary particles having a very small particle size of 0.01 to 0.04 .mu.m, said primary particles being able to give single particle size secondary particles with almost spherical in shape, and dense and desirable particle size by an improved treating method. In the non-oxide system, for example, Si.sub.3 N.sub.4 can be obtained by an imide method wherein primary particles are as small as 0.05 .mu.m or less. These fine primary particles grow by combining fine particles by a sintering treatment to large particles with uniform particle size depending on the sintering temperature and time. But it is inevitable to fuse the particles each other partly. Further, bridging easily takes place at the time of molding, which results in making it impossible to always produce products with high density.
According to the gas phase method, reaction gases previously mixed are introduced into a reaction zone, or reaction gases are directly mixed at the reaction zone. Since ceramic particles formed by synthesis or decomposition reaction have a melting point considerably higher than the temperature of the reaction zone, the growth of particles is difficult and the particle size obtained is about 0.01 .mu.m, which size is about 1/10 of the desirable particle size of 0.05 to 1.0 .mu.m. Although secondary particles may grow to some extent by collision of primary particles each other, it is difficult to control desirably the density, particle shape, particle size, and particle size distribution of the secondary particles. Thus, uniform dispersion of the particles at the time of molding is difficult, which results in failing to obtain dense molded products practically.
The gas phase method includes a plasma method, and particularly a hybrid plasma method, which are disclosed, for example, in Japanese Patent Unexamined Publication Nos. 55-32317 and 60-19034. According to these methods, one direct current (dc) arc plasma jet is combined with a high frequency induction plasma. The apparatus disclosed therein comprises a dc arc plasma torch and a high frequency induction coil, the central line of the both being owned jointly as shown in FIG. 5. According to these methods, there is a defect in that a starting material powder cannot be supplied to the high frequency induction plasma effectively.
The present inventors disclosed in Japanese Patent Unexamined Publication No. 60-77114 that spherical SiC having a particle size of 0.05 to 1.0 .mu.m with almost single particle size was synthesized by a plasma method using a Si compound and a carbon compound as starting materials. More concretely, a Si compound such as SiH.sub.4 is decomposed thermally at a temperature higher than the melting point of Si to form liquid particles, followed by reaction with a carbon compound such as CH.sub.4 gas at a temperature higher than the melting point of Si to give spherical SiC powder having a particle size of 0.1 to 1.0 .mu.m.
On the other hand, Japanese Patent Unexamined Publication No. 61-232269 discloses a process for producing B-containing SiC by introducing a carbon-free Si compound, or Si with a carbon-free boron compound or boron, yielding Si and B by reduction, pyrolysis or simple melting, making the temperature lower than the boiling point of Si but higher than the melting point of Si to form B-containing Si liquid spheres, followed by carbonization.
But according to these Japanese Patent Unexamined Publications, there is a problem in that the selection of starting materials is difficult. That is, when hydrogenated silicon such as SiH.sub.4 gas is used as a silicon compound, the desired Si liquid sphere can easily be formed by pyrolysis, but the hydrogenated silicon is expensive at present. When a chloride such as SiCl.sub.4 is used, Si or B can be obtained by reduction with hydrogen at a high temperature, but very corrosive HCl or Cl.sub.2 is produced to deteriorate the apparatus, which results in raising a problem of maintenance of the apparatus. Further, when Si and B powders are used to directly form liquid spheres of (Si +B) by melting, it is difficult to obtain a high purity powder of submicron size, the surface of which is not oxidized, or even if obtained, particles are aggregated undesirably at the time of blowing to form large (Si +B) liquid spheres having a particle size of 1 .mu.m or more. When vaporzied and passed as Si and B vapors, such aggregation does not take place. But very high temperature and remarkable temperature uniformity are required to vaporize the whole blown Si and B, so that in practice, aggregated particles pass a low-temperature portion without vaporization, a considerably large amount of particles are collected as large particles. This is a problem.
In the case of materials other than Si series materials, for example, Al, Zr, Mo, etc., production of hydrogenated compounds and access to these materials are sometimes very difficult.